Unconventional formations such as shale contain bedding planes and laminations along the horizontal direction. Such geological discontinuities influence acoustic wave velocities depending on the orientation of wave to the discontinuities. When an acoustic wave travels parallel to discontinuities in a formation, the wave velocity becomes faster. If the wave travels perpendicular to them, the wave speed is attenuated. In addition, in-situ stress conditions change wave velocities. Acoustic wave velocities can be investigated by a stress-dependent acoustic anisotropy test, which is considered a special, advanced rock physics experiment. In fact, it is important to measure acoustic wave velocities at various orientations and stress conditions for improving current formation evaluations, reservoir characterizations, horizontal drilling, and hydraulic fracturing technologies.
Past stress-dependent acoustic anisotropy test suffered from substantial limitations. First, specimen preparation has been restricted to preparing standardized specimens from shale formations. The conventional approach for plug sample specimens is to use a minimum of three plug specimens taken at the vertical, 45° and horizontal directions. FIGS. 1 and 2 illustrate prior art approaches where three plug specimens are taken from either a whole core (FIG. 1) or ⅔ butt section core (FIG. 2) to measure acoustic anisotropy along the vertical, 45° and horizontal directions. Each plug is used for a dynamic triaxial compressive test under various differential stresses. If a rock is a conventional rock such as sandstone, then the core specimen can allow getting all three plugs at an adjacent spot with the same depth and same lithology, because the condition of the rock is relatively homogeneous and isotropic.
This plug specimen approach can be problematic in anisotropic materials such as shale. In shale and other anisotropic materials, tight spacing of horizontal laminations does not allow having vertical and 45° specimens. Rather, it is usually impossible to get all plugs at the same spot. Thus, if plugs specimens are taken according to the above procedures, the plug specimens might be scattered along ±1 foot distance. Often, the testing plan needs to skip certain depths due to the lack of specimens.
Second, the current laboratory equipment for these tests relies on the assumption that measurements of ultrasonic wave velocity in each opposite direction must be equivalent. This assumption rests on the material being a continuum material; that is, the material not having microcracking, particle motion, and other defects that would make it anisotropic. However, this assumption is not generally true. Natural rocks are discontinuum materials because of the existence of various geologic discontinuities such as joints, faults, dykes, veins, bedding planes, laminations, foliations, gneissosity, shistosity, lineation, pores, voids, and other natural weaknesses within the rock.
To overcome the assumption of a continuum material, some approaches have proposed taking eight plug specimens from a whole core in order to study anisotropy with every 45°. As illustrated in FIG. 3, these approaches take eight short plugs of core specimens and the remaining core material is discarded. FIG. 3 shows the site selection for obtaining the plugs from a whole core (section). Thus, these approaches result in eight core specimens, each of which has to be tested. While the intent of this approach is good in terms of anisotropy measurement, this approach is often impractical or even unworkable for the reasons described above, i.e. it is usually impossible to get multiple plug specimens from the same spot for anisotropic or discontinuum materials, such as shale.
Accordingly, a system for performing a three-dimensional stress-dependent ultrasonic wave velocity test that overcomes the current limitations in terms of using an anisotropic specimen, especially one shale specimen per formation, and applying three-dimensional stress with different orientations would be advantageous.